Spark plugs are used to initiate combustion in internal combustion engines. Typically, spark plugs ignite a gas such as a fuel/air mixture in a combustion chamber by producing a spark across a spark gap, which is defined by two or more spark plug electrodes. The ignition of the gas by means of the spark produces a combustion reaction that in turn drives a piston inside an engine cylinder during a power stroke of the internal combustion engine.
The spark plug is generally accommodated in a spark plug socket of a cylinder head of the internal combustion engine. The spark plug socket has an internal thread. A fastening section of the spark plug is embodied with an externally threaded section so that the spark plug can be screwed into the spark plug socket of the cylinder head.
The general goal is to prevent exhaust gas from escaping during the combustion process. The escape of exhaust gas can occur if the exhaust gas flows along the thread engagement between the spark plug and the spark plug socket and out of the cylinder head of the internal combustion engine. In order to reduce or prevent a leakage of this kind, most spark plugs include a washer-like sealing ring. When the spark plug is installed in the spark plug socket of the cylinder head by means of thread engagement, the sealing ring is compressed between a shoulder of a metal sleeve of the spark plug and a cylinder head sealing surface. The compressed sealing ring then produces a gastight seal between the spark plug and the sealing surface of the cylinder head.
Depending on the design, spark plugs may be embodied with different thread types, for example M8, M10, M12, etc. In addition to different thread types, there are also different seat shapes. In this connection, the classic spark plug has a flat seat with the above-mentioned sealing ring. In addition to such a classic embodiment, however, there are also spark plugs with a conical seat that do not require a sealing ring (ISO 28741 or Wikipedia, keyword “spark plug” https://en.wikipedia.org/wiki/Spark_plug#Sealing_to_the_cylinder_head).
From document DE 10 2016 110 880 A1, it is known that the effectiveness of the gastight seal can be influenced by the material or composition of the sealing surface of the cylinder head. In order to reduce the weight of the vehicle, cylinder heads can be made of innovative production methods, for example a lost-foam casting process, in order to produce cylinder heads from light metal alloys. The cylinder blocks produced in this case have a more porous surface against which it can be difficult to achieve a seal. In addition, the challenge of producing a good seal can be made more difficult by the fact that during the installation process, the sealing ring is crushed or deformed in some other way so that a sealing surface thereof is no longer flat and planar.
In this connection, it is known from document DE 10 2016 110 880 A1 to slide a sealing ring over a fastening section of a metal sleeve of a spark plug and then to reshape the sealing ring into a post-formed [shape-adjusted] sealing ring, which has an inner diameter that is smaller than the outer diameter of the fastening section; the sealing ring that is reshaped in this way is flat and has an essentially uniform thickness.
U.S. Pat. No. 8,704,434 B2 discloses a spark plug with a conical sealing surface on the metal sleeve, which is designed to cooperate directly with a cylinder head sealing surface. In this case, protruding annular sections should be provided on the conical surface of the metal sleeve.
U.S. Pat. No. 9,422,911 B2 discloses a spark plug with a metal sleeve that has a sleeve sealing surface extending essentially perpendicular to a longitudinal axis. A sealing ring has sealing surfaces that are inclined. For installation in a cylinder head, this sealing ring is then compressed between sealing surfaces of a metal sleeve and a cylinder head, which are oriented perpendicular to a longitudinal axis.
U.S. Pat. No. 9,190,812 B2 discloses the production of a sealing ring on a spark plug in which the sealing ring is deformed against a sealing surface that is oriented perpendicular to the longitudinal axis in order to reduce the inner diameter of the sealing ring and affix the sealing ring to the metal sleeve in captive fashion.
Finally, U.S. Pat. No. 2,921,109 A discloses a spark plug with a sealing ring that is compressed between a sealing surface of a metal sleeve of the spark plug, which surface is oriented perpendicular to the longitudinal axis and a conical cylinder head sealing surface.